White dwarf
hideA white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth, it is very dense. Its faint luminosity comes from the emission of stored thermal energy. White dwarfs comprise roughly 6% of all known stars in the solar neighborhood. The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering, and Williamina Fleming;, p. 1 the name white dwarf was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our galaxy., §1. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. (SN 1006 is thought to be a famous example.)
A white dwarf is very hot when it is formed but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer be visible, and become a cold black dwarf. However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.
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News tagged with white dwarfs
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Nov 17, 2009 |
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(PhysOrg.com) -- "One of the major problems in modern astrophysics is the fact that we still do not know exactly what kinds of stellar system explode as a Type Ia supernova," says Patrick Woudt, from the University ...
Two Earth-sized bodies with oxygen rich atmospheres found -- but they're stars not planets
Nov 12, 2009 |
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(PhysOrg.com) -- Astrophysicists at the University of Warwick and Kiel University have discovered two earth sized bodies with oxygen rich atmospheres - however there is a bit of a disappointing snag for anyone ...
New type of supernova explosion reported; predicted by theoretical physicists at UCSB
Nov 05, 2009 |
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A new class of supernova was discovered by scientists at Berkeley and may be the first example of a new type of exploding star. A team of astrophysicists at UC Santa Barbara had predicted this kind of explosion in their t ...
Rapid supernova could be new class of exploding star
Nov 05, 2009 |
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(PhysOrg.com) -- An unusual supernova rediscovered in seven-year-old data may be the first example of a new type of exploding star, possibly from a binary star system where helium flows from one white dwarf ...
German high-school students involved in an astronomical research project
Space & Earth / Space Exploration
Nov 05, 2009 |
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This week, Astronomy & Astrophysics publishes a somewhat unusual research article because it is co-authored by German high-school students. Led by astronomer Klaus Beuermann (University of Göttingen, Germany), the team i ...
The Ring Nebula
Nov 02, 2009 |
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The diversity of colours, shapes, and sizes of planetary nebulae make them fascinating objects. In this photo release Calar Alto presents a rather unique view combining both optical and near-infrared data ...
Computer code gives astrophysicists first full simulation of star's final hours
Sep 22, 2009 |
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The precise conditions inside a white dwarf star in the hours leading up to its explosive end as a Type Ia supernova are one of the mysteries confronting astrophysicists studying these massive stellar explosions. But now, ...
XMM-Newton uncovers a celestial Rosetta stone
Sep 03, 2009 |
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(PhysOrg.com) -- ESA's XMM-Newton orbiting X-ray telescope has uncovered a celestial Rosetta stone: the first close-up of a white dwarf star, circling a companion star, that could explode into a particular ...
Variability of type 1a supernovae has implications for dark energy studies
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(PhysOrg.com) -- The stellar explosions known as type 1a supernovae have long been used as "standard candles," their uniform brightness giving astronomers a way to measure cosmic distances and the expansion ...
Pulsating White Dwarfs Explained by 'Accidental Astronomer'
Space & Earth / Space Exploration
Jul 28, 2009 |
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(PhysOrg.com) -- Exploring distant parts of the galaxy, astrophysicist Denis Sullivan has collaborated on the discovery of about six extrasolar planets--not bad for an accidental astronomer.
Peculiar, junior-sized supernova discovered by New York teen
Jun 11, 2009 |
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In November 2008, Caroline Moore, a 14-year-old student from upstate New York, discovered a supernova in a nearby galaxy, making her the youngest person ever to do so. Additional observations determined that the object, called ...
Supernova remnant is an unusual suspect
Jun 09, 2009 |
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A new image from NASA's Chandra X-ray Observatory shows a supernova remnant with a different look. This object, known as SNR 0104-72.3 (SNR 0104 for short), is in the Small Magellanic Cloud, a small neighboring ...
Queen's astronomers propose new supernova interpretation
Jun 09, 2009 |
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In a controversial new paper in the journal Nature, astronomers from Queen's University Belfast have proposed a new physical interpretation of a supernova discovered on 7th November 2008.
World's observatories watching 'cool' star
May 15, 2009 |
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The Whole Earth Telescope (WET), a worldwide network of observatories coordinated by the University of Delaware, is synchronizing its lenses to provide round-the-clock coverage of a cooling star. As the star ...
Galactic X-ray emissions originate from stars
May 05, 2009 |
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(PhysOrg.com) -- A 25-year old astronomical mystery has been solved: Most of the diffuse X-ray emissions in the Milky Way do not originate from one single source but from so-called white dwarfs and from stars ...
